1. Field of the Invention
The present invention relates to a work vehicle having a hydraulic stepless speed changing apparatus and a gear type speed changing apparatus.
2. Description of the Related Art
From the Japanese Patent Application “Kokai” No. 2004-257447, there is known, as an example of the above-noted work vehicle, a multi-purpose work vehicle configured such that engine power is inputted to a main speed changing apparatus to be speed-changed in a stepless manner, and the resultant speed-changed power in a predetermined rotational direction is transmitted to a gear type auxiliary speed changing apparatus to be converted selectively into a forward driving power or a reverse driving power in plurality of stages of forward/reverse speeds, respectively. With this work vehicle, when the any speed changing apparatus is to be switched over, the speed changing operation is effected after a traveling brake is activated for braking. However, if the auxiliary speed changing operation is effected during e.g. a slope traveling when there is a significant pressure difference between the high pressure side oil passage and the low pressure side oil passage, a gear speed changing operation needs to be effected, with the torque from the main speed changing apparatus being applied to the auxiliary speed changing apparatus. Therefore, the shifting operation cannot sometimes be effected smoothly.
In an attempt to solve the above problem, in the Japanese Patent Application “Kokai” No. 2007-239978 (paragraphs [0032] to [0036], FIG. 8 to FIG. 10) assigned to the present applicant, there was proposed a technique as follows. In this, a bypass passage is formed between a high-pressure side circuit and a low-pressure side circuit of a hydrostatic stepless speed changing apparatus as a main speed changing apparatus. The bypass passage incorporates therein an opening/closing valve and a manual operating member is provided for switching over the opening/closing valve. However, with this conventional art, if bypass between the high-pressure side circuit and the low-pressure side circuit is established by an operation of the operating lever simultaneously with a stepping operation of the brake pedal to the braking side, the torque of the hydrostatic stepless speed changing apparatus is released all at one time, thus resulting in sudden release of the torque of the hydrostatic stepless speed changing apparatus This release of torque results in sudden release of the braking function of the hydrostatic stepless speed changing apparatus. As a result, the work vehicle becomes unstable until complete application of a braking force on the slope or the like.